1. Field of the Invention
The present invention relates to the solvent separation or rare earth values from aqueous solutions thereof, and, more especially, to the liquid/liquid selective extraction of rare earth values from aqueous solutions of the nitrates of such values.
By the expression "rare earths" as utilized herein, there are intended the lanthanide rare earth elements having atomic numbers of from 57 to 71, inclusive, as well as yttrium which has an atomic number of 39.
Also as utilized herein, by the expression "ceric rare earths" are intended the lightest of the rare earth elements, beginning with lanthanum and extending to neodymium according to atomic number, and by "yttric rare earths" are intended the heavier rare earth elements based on atomic number, beginning with samarium and terminating with lutecium and including yttrium.
2. Description of the Prior Art
It is well known to this art that the rare earths are very difficult to separate from each other because of the similarity in their properties.
In order to separate these elements individually, which are very closely related, methods have to date been developed which have become particularly valuable in the separation of the rare earths, namely, various liquid/liquid extraction techniques. Such techniques are based upon the selective extraction of one of the rare earths from solution thereof by means of an extraction solvent, or extractant, which is immiscible with the solution.
Certain organophosphoric acids, for example, have heretofore been used as extractants for the separation of the rare earths by reason of their marked selectivity, both with respect to the ceric rare earths, as well as with respect to the yttric rare earths.
Thus, diesters of phosphoric acid, in particular di-(2-ethylhexyl)phosphoric acid or H.D.E.H.P. (see published French Patent Application No. 2,233,284) are currently used as extraction solvents. However, militating against their good selectivity is the fact that extraction with such extractants requires large amounts of reagents upon the separation of the yttric rare earths, due to the difficult re- or back-extraction at the point in time of regeneration of the solvent. And energy costs are at times increased due to the necessity of concentrating the solutions which are obtained in dilute form upon re-extraction for the same reason.
The art was advanced by using esters of alkylphosphonic acids, in particular the 2-monoethylhexyl ester of 2-ethylhexylphosphonic acid or H.E.P. (E.H.P.) (see published French Patent Application No. 2,460,275), which esters permit more economical separations due to a low extraction constant with, hence, an easier re-extraction. Nevertheless, the aforementioned drawbacks are again present, although to a lesser extent, in the case of the separation of the yttric rare earths, which are the heaviest.
Another technique for reducing the value of the extraction constant consists of replacing the diluents customarily used, i.e., aliphatic hydrocarbons such as kerosene or dodecane, by aromatic hydrocarbons such as, for example, xylene or Solvesso. Such technique, however, has its drawbacks since these latter diluents are less resistant to degradation by the various chemical agents to which they are exposed.